Casting mold composition with improved detectability for inclusions and method of casting / General Electric Company

Title: Casting mold composition with improved detectability for inclusions and method of casting.Abstract: The present disclosure relates to a titanium-containing article casting mold composition comprising calcium aluminate and an X-ray or Neutron-ray detectable element. Furthermore, present embodiments teach a method for detecting sub-surface ceramic inclusions in a titanium or titanium alloy casting by combining calcium aluminate, an element more radiographically dense than the calcium aluminate, and a liquid to form a slurry; forming a mold having the calcium aluminate and the radiographically dense element from the slurry; introducing a titanium aluminide-containing metal to the radiographically dense element-bearing mold; solidifying said titanium aluminide-containing metal to form an article in the mold; removing the solidified titanium aluminide-containing metal article from said mold; subjecting the solidified titanium aluminide-containing article to radiographic inspection to provide a radiograph; and examining said radiograph for the presence of the radiographically dense element on or in the article. ...

BACKGROUND

Modern gas turbines, especially aircraft engines, must satisfy the highest demands with respect to reliability, weight, power, economy, and operating service life. In the development of aircraft engines, the material selection, the search for new suitable materials, as well as the search for new production methods, among other things, play an important role in meeting standards and satisfying the demand.

The materials used for aircraft engines or other gas turbines include titanium alloys, nickel alloys (also called super alloys) and high strength steels. Titanium alloys are generally used for compressor parts, nickel alloys are suitable for the hot parts of the aircraft engine, and the high strength steels are used, for example, for compressor housings and turbine housings. The highly loaded or stressed gas turbine components, such as, components for a compressor, for example, are forged parts. Components for a turbine, on the other hand, are typically fabricated as investment cast parts.

Although investment casting is not a new process, the investment casting market continues to grow as the demand for more intricate and complicated parts increase. Because of the great demand for high quality, precision castings, there continuously remains a need to develop new ways to make investment castings more quickly, efficiently, cheaply and of higher quality.

Conventional investment mold compounds that consist of fused silica, cristobalite, gypsum, or the like, that are used in casting jewelry and dental prostheses industries are not suitable for casting reactive alloys, such as titanium alloys. One reason is because there is a reaction between mold titanium and the investment mold. It is difficult to investment cast titanium and titanium alloys and similar reactive metals in ceramic molds because of the titanium's high affinity for elements such as, oxygen, nitrogen, and carbon. At elevated temperatures, titanium and its alloys can react with the mold facecoat.

The properties of the final casting are greatly deteriorated if any reaction occurs between the molten alloy and the mold. The form of this deterioration can include a poor surface finish due to gas bubbles, or in more serious cases, the chemistry, microstructure, and properties of the casting are compromised. Asperities and/or pits on the surfaces of cast alloy components can reduce aerodynamic performance in, for example, turbine blade applications, and increase wear and friction in rotating or reciprocating part applications. Therefore, there is a need in the art for new, practical and useful casting mold compositions and methods for detecting inclusions in reactive alloys, such as titanium alloys.

SUMMARY

Aspects of the present disclosure provide casting mold compositions, methods of casting, and cast articles that overcome the limitations of the state of the art. Some aspect of the disclosure may be directed toward the fabrication of components for the aerospace industry, for example, engine turbine blades. Further aspects may be employed in the fabrication of a component in any industry, in particular, those components containing titanium and/or titanium alloys.

One aspect of the disclosure is a mold composition for casting a titanium-containing article, comprising: a calcium aluminate cement comprising calcium monoaluminate, calcium dialuminate, and mayenite; and an X-ray or Neutron-ray detectable element. Another aspect of the present disclosure is a titanium-containing article casting-mold composition, comprising: calcium aluminate; and an X-ray or Neutron-ray detectable element. In one embodiment, the calcium aluminate cement forms an intrinsic facecoat of less than about 100 microns when the mold composition forms a mold. In one embodiment, the X-ray or Neutron-ray detectable elements are mixed within the mold. In another embodiment, the X-ray or Neutron-ray detectable elements are mixed within the mold and become part of the intrinsic facecoat. In one embodiment, the mold composition does not have an extrinsic facecoat.

In another embodiment, X-ray or Neutron-ray detectable element comprises ytterbium, hafnium, gadolinium, tungsten, thorium, uranium, yttrium, dysprosium, erbium, cerium, and compositions thereof. The X-ray or Neutron-ray detectable element may be in the range of about 1 to about 4 weight percent in the mold composition. The radiographically dense element may be more radiographically dense than the oxide particles. In one embodiment, radiographically dense element is more radiographically dense than calcium aluminate and comprises one or more of ytterbium, hafnium, gadolinium, tungsten, thorium, uranium, yttrium, dysprosium, erbium, cerium and compositions thereof.

One aspect of the present disclosure is a method for detecting sub-surface ceramic inclusions in a titanium or titanium alloy casting, said method comprising: combining calcium aluminate, at least one element more radiographically dense than the calcium aluminate, and a liquid to form a slurry; forming a mold having the calcium aluminate and the radiographically dense element from the slurry; introducing a titanium aluminide-containing metal to the radiographically dense element-bearing mold; solidifying said titanium aluminide-containing metal to form an article in the mold; removing the solidified titanium aluminide-containing metal article from said mold; subjecting the solidified titanium aluminide-containing article to radiographic inspection to provide a radiograph; and examining said radiograph for the presence of the radiographically dense element on or in the article.

In one embodiment, the method further comprises removing the radiographically dense element from the article. The removing the radiographically dense element from the article may comprise one or more steps of machining, grinding, polishing, or welding. The combining step may further comprise combining oxide particles with the slurry. In one embodiment, the oxide particles comprise hollow oxide particles, for example, hollow aluminum oxide particles.

In one embodiment, the method comprises minimizing the presence of mold material inclusions in titanium aluminide-containing cast articles. The titanium-containing cast article may comprise an engine or turbine, or a component of a turbine. For example, the titanium-containing cast article comprises a turbine blade. The titanium-containing cast article may be a titanium aluminide containing engine, a titanium aluminide containing turbine, or a titanium aluminide containing turbine blade.

One aspect of the present disclosure is a mold composition comprising: calcium aluminate cement comprising calcium monoaluminate, calcium dialuminate, and mayenite; and at least one element more radiographically dense than the calcium aluminate cement. Another aspect of the present disclosure is a mold composition comprising calcium aluminate and at least one element more radiographically dense than the calcium aluminate. In one embodiment, the mold composition further comprises oxide particles. In a related embodiment, the radiographically dense element is further more radiographically dense than the oxide particles.

Another aspect of the present disclosure is a mold composition for casting titanium-containing articles, comprising: calcium aluminate; and an X-ray or Neutron-ray detectable element. For instance, an aspect of the present disclosure may be uniquely suited to providing mold compositions to be used in molds for casting titanium-containing and/or titanium alloy-containing articles or components, for example, titanium containing turbine blades.

These and other aspects, features, and advantages of this disclosure will become apparent from the following detailed description of the various aspects of the disclosure taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES

The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features and advantages of the present disclosure will be readily understood from the following detailed description of aspects of the disclosure taken in conjunction with the accompanying drawings in which:

FIG. 1 is a diagram that depicts the percentage of aluminum oxide on the x axis and temperature on the y axis, showing various calcium oxide-aluminum oxide composition ranges for the calcium aluminate cements, and shows particular aluminum oxide percentages and temperature ranges for the compositions according to disclosed embodiments.

FIGS. 2a and 2b show one example of the mold microstructure after high temperature firing. The backscattered scanning electron microscope images of the cross section of the mold fired at 1000 degrees Celsius are shown, wherein FIG. 2a points to the alumina particles present and FIG. 2b points to the calcium aluminate cement.

FIG. 3a and FIG. 3b show one example of the mold microstructure after high temperature firing. The backscattered scanning electron microscope images of the cross section of the mold fired at 1000 degrees Celsius are shown, wherein FIG. 3a points to calcium aluminate cement and fine-scale alumina particles present and FIG. 3b points to an alumina particle.

FIG. 4a and FIG. 4b show on example of the mold microstructure after high temperature firing. The backscattered scanning electron microscope images of the cross section of the mold fired at 1000 degrees Celsius are shown, wherein FIG. 4a points to a large scale alumina particle and FIG. 4b points to a calcium monoaluminate particle.

FIG. 5 shows one example of the mold microstructure after high temperature firing, showing alumina and calcium monoaluminate, wherein the calcium monoaluminate reacts with alumina to form calcium dialuminate, and wherein the mold in one example is fired to minimize mayenite content.

FIG. 6 shows one example of the mold microstructure after high temperature firing, showing alumina and calcium monoaluminate, wherein the calcium monoaluminate reacts with alumina to form calcium dialuminate, and wherein the mold is fired to minimize mayenite content.

FIG. 7 shows X-ray images in planar view of a cast titanium aluminide article. FIG. 7a shows an X-ray image, with arrows pointing to examples of sub-surface inclusions and casting porosities. FIG. 7b is an zoomed in view of FIG. 7a. FIG. 7b shows an example of a sub-surface inclusion from the mold that is 5.44 mm in length. Casting porosities are also indicated, with one example the diameter of the porosity is indicated to be 0.99 mm.

FIG. 8 shows a schematic of the mold with the facecoat. FIG. 8a shows the mold with the intrinsic facecoat that is approximately 100 microns thick. The schematic shows the intrinsic facecoat with the mold cavity and calcium aluminate mold positions also indicated. FIG. 8b shows the mold with the extrinsic facecoat that is approximately 100 microns thick. The schematic shows the extrinsic facecoat with the mold cavity and calcium aluminate mold positions also indicated.

FIG. 9 shows a flow chart, in accordance with aspects of the disclosure, illustrating the steps of a method for detecting sub-surface ceramic inclusions in a titanium or titanium alloy casting.

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